In a sensorless control for driving a brushless DC (Direct Current) motor in a sensorless manner, rotation synchronization control is performed by detecting a zero-cross signal with a neutral point of a back electromotive force (for example, see PTL 1).
During activation in the sensorless control, phase alternation is performed with a constant rate, and a zero-cross signal is detected, so that the brushless DC motor is caused to follow and the brushless DC motor is activated.
When the oil temperature is low in the oil pump motor, the viscosity of oil is high, and therefore, the rotation speed is limited because of reasons such as cavitation. When the rotation speed is low, the back electromotive force is small, and therefore, it is difficult to detect the zero-cross signal.
It has been proposed that the rotation control is performed with a constant rotation rate at a low temperature in accordance with the oil temperature, and when the oil temperature attains a high temperature, or when a certain period of time passes, synchronization is carried out (for example, see PTL 2).